After you have confirmed the other three cryptids (Nessie, Chupacabra, and the Jersey Devil), and brought back the inconclusive footprint from the Himalayas, Mews will send you in pursuit of Bigfoot, which is similar to the Yeti.
Since you have him in a cage at the end (but let him out), he is definitely confirmed.
You get the Island Medallion after you prove the existence of three of the four cryptids, and rescue Bigfoot from the evil Gretchen Grimlock. (see related question)
first you have to find all the other cryptids then you go in the helicoptor to the new location and follow him to his cave
There are 3 you have to prove: Nessie (Loch Ness) Chupacabra (Puerto Rico) New Jersey Devil (New Jersey) You can't get what you need to prove the Yeti, so you are sent to find Bigfoot.
After you only get a footprint photo from the Himalayas, Mews decides that it is too difficult to track the Yeti. When you prove the other 3 cryptids, he sends you in search of Bigfoot, and you have to rescue the creature from the evil Gretchen Grimlock.
You still have to prove the chupacabra and the Jersey Devil. You can get a photo from the Himalayas, but it does not prove the Yeti. So when you have the other three proven, you will be sent to chase Bigfoot in the Northwest.
You will find a rope inside and you can get the egg shells from the nest of the Jersey Devil. Then you can prove it exists. You still need to get the photo from the Himalayas and also prove the Loch Ness Monster and the chupacabra. Once the three are proven, you will be sent to chase Bigfoot instead of the Yeti.
You can't, because it never appears. You do find Bigfoot at the end of the quest. All you can do at the Himalayas is get the lantern from the monastery and a photo of a footprint high up on the mountain. This is not enough proof. When you prove the other three cryptids, Mews will send you after Bigfoot instead.
Prove 4 cryptids to qualify for the Mews million-dollar reward. You find a footprint that may be from the Yeti, but you can only prove Nessie, the Chupacabra, and the New Jersey Devil. Then you have to track Bigfoot and save him from Gretchen Grimlock. (see the related solution question)
Many people indeed believe in cryptids. For example, the giant squid. By proving the giant squid exists more people believed then before. Some people are still trying to prove the existens of cryptids. Some examples are, bigfoot, nessy, the chubacabra, and the jersey devil.
Once you have gotten the lantern and the footprint photo from the Himalayas, you still cannot prove the Yeti. You can prove the other three cryptids (Nessie, the chupacabra, the Jersey Devil). Once you do, a fifth location will appear on the helicopter controls: you must go to the Northwest to chase Bigfoot.
You cannot prove the Yeti based on the evidence from the Himalayas. So you have to prove the other 3 (Nessie, Chupacabra, and New Jersey Devil) and when you do, Mews will send you to pursue Bigfoot in the Pacific Northwest.
You cannot prove the Yeti. But after you confirm the other three cryptids (Nessie, Chupacabra, and Jersey Devil), Mews will send you in pursuit of the similar creature, Bigfoot, in the Pacific Northwest. The location will be added to the GPS on the helicopter.